1. Field of the Invention This invention concerns a non-volatile semiconductor device constituting a non-volatile semiconductor memory, especially an EPROM (Erasable PROM) and an EEPROM (Electrical Erasable PROM).
2. Description of the Prior Art
In general, mobile ions in a device cause deterioration of the charge retention characteristics and instability of the threshold voltage of the device.
Recently, non-volatile memories have been used in various fields and occasionally under relatively high temperature conditions. Thus, it is required to restrict the amount of mobile ions in the device to as small an amount as possible.
Conventionally, mobile ions such as sodium ions Na.sup.+, which adhere to a wafer during wafer processing, are fixed in a PSG (Phospho-Silicate Glass) layer used in a gettering process. For example, as shown in FIG. 1, a PSG layer 6 is formed on an oxide layer 5, and a high temperature treatment is carried out. In the temperature treatment, mobile ions adhered to the wafer move into the PSG layer 6, and are fixed therein. Furthermore, the PSG layer 6 acts to prevent invasion of mobile ions from the exterior.
However, in the conventional device, the invasion of mobile ions from the contact hole 7 for e.g., a source region 8 to a memory cell section 2 having a floating gate electrode 3 and a control gate electrode 4, can not be avoided. This occurs because the contact hole 7 is formed after the gettering process, and the oxide layer 5 does not act as a barrier layer against the invasion of the mobile ions, since the mobile ions move in the oxide layer. Furthermore, after the formation of the contact hole, there is no processing step performed at high temperatures. Thus, the mobile ions are not fixed in the PSG layer 6. Therefore, in the latest LSI devices, the invasion of mobile ions is mainly caused by the contact hole.